Digital geometric-sequence control technique for bidirectional dual active bridge DC-DC converters used in future electric vehicles

Bidirectional DC-DC converters are widely used in different applications such as energy storage systems, Electric Vehicles (EVs), UPS, etc. In particular, future EVs require bidirectional power flow in order to integrate energy storage units into smart grids. These bidirectional power converters provide Grid to Vehicle (V2G)/ Vehicle to Grid (G2V) power flow capability for future EVs. Generally, there are two control loops used for bidirectional DC-DC converters: The inner current loop and The outer loop. The control of DAB converters used in EVs are proved to be challenging due to the wide range of operating conditions and non-linear behavior of the converter. In this thesis, the precise mathematical model of the converter is derived and non-linear control schemes are proposed for the control system of bidirectional DC-DC converters based on the derived model. The proposed inner current control technique is developed based on a novel Geometric-Sequence Control (GSC) approach. The proposed control technique offers significantly improved performance as compared to one for conventional control approaches. The proposed technique utilizes a simple control algorithm which saves on the computational resources. Therefore, it has higher reliability, which is essential in this application. Although, the proposed control technique is based on the mathematical model of the converter, its robustness against parameter uncertainties is proven. Three different control modes for charging the traction batteries in EVs are investigated in this thesis: the voltage mode control, the current mode control, and the power mode control. The outer loop control is determined by each of the three control modes. The structure of the outer control loop provides the current reference for the inner current loop. Comprehensive computer simulations have been conducted in order to evaluate the performance of the proposed control methods. In addition, the proposed control have been verified on a 3.3 kW experimental prototype. Simulation and experimental results show the superior performance of the proposed control techniques over the conventional ones.

‘Black Settlers’: Hybridity, Neoliberalism and Ecosystemic Change Among Tonga Farmers of Southern Zambia, 1964-2008

This dissertation examines a process of indigenous accumulation among Tonga farmers in Zambia’s Southern Province. In the 1970s multiple authors concluded that capitalist farmers had emerged among Tonga agro-pastoralists, predominantly within private titled holdings. Relying on archival research, newspapers, secondary sources and extensive oral testimony this thesis fills a 35-year gap on the topic, providing insights into the social and environmental impacts of neoliberal policy among African peasants and capitalist farmers. In contrast to dominant narratives of the post-independence period, this study argues that Zambia did experience a developmental process post-independence, which saw significant achievements made in the agricultural sector, including the doubling of national cattle stocks. The data reveals a painful process of disarticulation beginning in the late 1980s. Following neoliberal adjustment, we observe significant heterogeneity in production systems, some regional specialization, and processes of migration. Most importantly, the thesis uncovers processes of overwhelming ecosystemic change that contributed to livestock epidemics of severe scale and scope. Amazingly, this went largely undocumented because of the simultaneous crisis of the state, which left the national statistics office and other state bodies incapable of functioning from the late 1980s into the 2000s. In response, the Zambian state has introduced a number of neodevelopmental initiatives in the sector, yet the lack of animal traction remained up to 2008 and agricultural production declined, while more capitalized farmers (largely white, and/or with foreign direct investment) have become more significant players in the country. This thesis provides compelling evidence to challenge dominant economic thinking of the Washington institutions as well as many of the common Marxian formulations.